The invention relates to a method for continuous production of a nonwoven provided with holes in which the unperforated nonvoven is subjected to a hydrodynamic needling in which the holes are produced by compression of fibers and the nonwoven is then subjected to an at least partial drying.
Hole patterns can be produced in accordance with U.S. Pat. No. 3,750,237. Then the prefabricated nonwoven, held between two endless webs, is struck radially from the outside by hard water jets to produce a hole structure. The device consists of a uniformly perforated drum covered all the way around by an endless screen. The endless screen has open and closed areas depending on the desired hole structure. The disadvantage of this method of producing the holes is the fact that no holes with sharply delimited edges can be produced in this fashion and in addition individual nonwoven fibers are displaced toward the drum by the hard water jets as the holes are produced.
Sharply delimited holes can be produced subsequently in a prefabricated uniform nonwoven using the manufacturing methods according to EP-A-0 215 684, 0 223 614, or 0 273 454. In each case, a perforated drum made of smooth sheet metal is produced with drainage openings on which plastic elevations uniformly distributed over the surface between the openings are formed. The plastic elevations can consist of beads open half way so that the drainage openings are formed at the same time or even better from uniformly distributed mandrels tapering upward to a point between which holes are made in the sheet metal as drainage openings. The water jets strike this drum surrounded by the nonwoven radially from the outside. In all cases, the drum is made of a metal sheet to which the mandrels or other plastic elevations can simply be screwed; see also U.S. Pat. No. 3,034,180.
Practice has shown that depending on the fibers used for the nonwoven, holes with clean edges can be produced with difficulty in a previously uniform nonwoven, holes that have no fibers stretched across the hole-like openings in the nonwoven.
The goal of the invention is to provide a method and an advantageous device for working this method with which this clean hole edge structure can be produced in a water-needled nonwoven.
On the basis of the conventional method for hydrodynamic needling of a nonwoven on a drum with plastic elevations thereon, the invention provides as the solution to the problem that the nonwoven provided with holes is subjected to singeing flames or subsequent destruction of any transversely stretched individual fibers still present over the cross section of the holes. If stiffer fibers are to be contained in the nonwoven, which can be displaced at least not permanently into the marginal areas of the respective holes by the water needling on a perforated drum, these fibers, namely only the ones that are stretched across the holes, are subjected briefly to melting. Ends of these individual fibers separated by melting retract automatically into the marginal areas of the holes so that neater holes can be produced continuously in this fashion by using water needling.
It is advisable to cool the nonwoven during singeing, or rather directly afterward, so that other fibers in the nonwoven will not be heated unnecessarily. It can also be advantageous to allow the flames of the singeing device to pass through the previously formed holes. For this purpose, the nonwoven to be singed does not have to rest on the substrate but can be guided hanging freely and then cooled again immediately afterward. One can also think of subjecting the flames on the opposite side of the nonwoven to suction so that they reliably act through the holes to melt the fibers held stretched there.